Sample channel processors are frequently used in signal processing circuits to enable accurate reading of high frequency signals of devices such as communication channels (modems), disk drive read channels, CD ROMs, and recording channels. These systems essentially consist of an encoder/transmitter which receives data input comprising a series of state transitions, and the channel which receives the encoded data input and frequently introduces unwanted distortions and noise. The channel output is delivered to a filter which removes noise and samples the encoded signal, after which a detector determines whether a signal transition has occurred based on the samples taken from the filter, and a decoder provides data output based on the detected signal. For successful data transmission, the data output should be the same as the data input. The effectiveness of the data transmission depends on how accurately the sampled data represents the actual input data signal.
As technological advances enable devices to operate at increased data rates, the transitions occur closer together in time, making it more difficult to filter out channel noise and retain the integrity of the original input data signal based on the data samples taken. A number of prior art methods have been developed to overcome this problem.
One simple approach to overcoming this problem is to use a Decision Feedback Equalizer (DFE). A DFE uses one data sample to determine whether or not a transition has occurred in the input data signal. A DFE circuit essentially consists of a filter, an adder, a detector (usually a comparator) and a feedback equalizer. The filter concentrates the energy of the input signal so that the amplitude of the signal exceeds a predetermined detection threshold, and takes one sample from the incoming signal. The remaining signal information is discarded. The comparator looks at the amplitude of this truncated sample signal and detects whether or not the sample has exceeded the predetermined threshold, indicating that a state transition has occurred. The feedback equalizer responds to the output of the detector, adding a feedback signal to the input of the comparator, thus incorporating signal information from the previous sample into the processing of the current sample. Problems with this technique include loss of important signal information because of the reliance on only one sample and distortion of the input signal.
Yet another problem arises from the recursive or feedback nature of the decision which uses past decisions to generate optimal feedback equalizer output to cancel post-cursor intersymbol interference (ISI). But this optimal decision is made only when all past decisions are correct. If any of them were in error there is a likelihood that the feedback equalizer will generate an output which instead of cancelling post cursor ISI will actually increase or compound the error by feeding back cancellation signals of the wrong polarity. As a result further errors, burst errors, can be generated by the DFE detector. Thus such systems are capable of generating a long sequence of burst errors in response to a single bit error. The duration of this error sequence is characterized by the burst error length measured by the number of bits that may contain faulty decisions. The theoretical probability of the occurrence of any particular burst error length is inversely proportional to the length. There has to be a limit in a practical system so as not to exceed the burst error correction capabilities of presently used error correction methods. For example, in current disc drive channels the probability of a burst error which is longer than 60 bits has to be kept below one in ten thousand. Prior art systems do not meet this target.